# Artificial outdoor light at night and depression in older adults in the USA, England, Northern Ireland, and Ireland

## TL;DR

This study finds that exposure to artificial outdoor light at night is linked to higher rates of depression in older adults across four countries.

## Contribution

The study provides cross-national evidence linking artificial light at night to depression in aging populations.

## Key findings

- Higher artificial light at night exposure was associated with increased depression prevalence in older adults.
- The strongest association was observed in the US and Ireland, with prevalence ratios up to 1.79 in Northern Ireland.
- Associations remained after adjusting for air pollution (NO2).

## Abstract

Artificial Light at night (ALAN) is a potential environmental stressor for depression, but epidemiological evidence is limited. Cross-national surveys of aging were leveraged to examine LAN and depression.

We used longitudinal aging surveys from the US (HRS; n = 20,868), England (ELSA; n = 9848), Ireland (TILDA; n = 6407), and Northern Ireland (NICOLA; n = 2725). Depression was ascertained using Center for Epidemiologic Studies Depression Scale and dichotomized based on study-specific cutoffs. Annual mean outdoor ALAN exposure was estimated using satellite-derived nighttime light data (~500 m resolution), then categorized using harmonized quartiles, based on cross-country population values (≤2.79, 2.79-11.69, 11.69-23.93 and >23.93 nW/cm2/s). Poisson regression models estimated the prevalence ratios (PRs) of depression, adjusting for individual- and area-level factors.

The prevalence of depression was highest in ELSA and HRS (24%), followed by NICOLA (14%) and lowest in TILDA (8%). The mean (SD) ALAN levels were 18.9 (9.0) nW/cm2/s in HRS, 13.4 (12.7) in ELSA, 10.4 (13.4) in TILDA, and 11.6 (10.3) in NICOLA. In fully-adjusted models, the highest ALAN quartile was associated with higher PRs of depression (reference: lowest quartile), in all surveys, with PRs (95% Confidence interval) of 1.40 (1.20–1.63) in HRS, 1.16 (0.98–1.38) in ELSA, 1.51 (1.08–2.10) in TILDA, and 1.79 (1.13–2.84) in NICOLA. The directions of the association were robust to adjustment for NO2, though attenuated.

Findings from multiple countries suggest that outdoor ALAN exposure is associated with depression in older adults and highlight the value of international longitudinal aging cohorts for investigating the impact of environmental exposures on health.